Appyling Pre-Emergents in Stubble

There are a range of benefits when it comes to using pre-emergent herbicides within your integrated weed management strategy for the year.

The core benefits of pre emergent herbicides includes: 

1# Pre-emergent’s offer an alternative mode of action

2# Reduces the selection pressure on post emergent herbicides

3# Eliminates the early season weed burden

4# Cost savings in the fallow where multiple knock downs may be needed

5# Requires a cultivation which encourages weed germination within the herbicide zone

6# Provides an opportunity in crops with few post emergent options

The article will aim to give farmers an indication of how pre-emergent herbicides act upon the plant, breakdown in the soil and how they interact with stubble’s in stubble retained systems.

When herbicides are applied pre-emergent its important to account of the environmental impacts upon the herbicide and how they impact its interaction with the target weed species. An example of this would be relatively insoluble herbicides that bind tightly to soil colloids. Even though these herbicides are unlikely to leach environmental conditions may provide the optimal opportunity for leaching to occur. If the soil is relatively dry and a large rainfall event occurs it is possible that the highly insoluble herbicide will move further down into the soil profile. This means that the herbicide will no longer remain in the zone where it is effective for weed control.

Interactions with stubble

Stubble’s in a minimum till system are capable of intercepting the pre-emergent herbicide that’s applied before it reaches the soil. It is found that the amount of herbicide that is intercepted is proportionate to the percentage of ground cover.

stubble interception

Sourced from the GRDC

The negative effects of the herbicide being caught within the stubble canopy are two fold.

  • Some products require incorporation or soil contact in order to activate. This leads to ineffective weed control later on.
  • Interception may also lead to an uneven coverage of the soil surface, which results in insufficient herbicide being available for effective control of the weed population.

When the herbicide comes into contact with standing organic material the herbicide is subject to a degree of binding which is dependent on the herbicide and its characteristics. In some cases herbicides are tightly bound and as a result are lost to the system in relation to weed control even though a subsequent rainfall event may occur. Other herbicides are only loosely bound and can be returned into the system and still impact upon the weed population. Even though some herbicides are able to be washed back into the system from the stubble, the herbicide may be subject to volatility and photo degradation.

What can I do?

Under conditions of high stubble loads the degree of spray droplet interception can be minimised by simply adjusting how the herbicide is applied.

Some techniques that can maximise the levels of herbicide reaching the soil include:

  • Wind across the rows during application
  • Use rear facing nozzles where the angle is able to offset the travel speed. The aim is to have the droplets moving predominately downwards through the stubble
  • Select a nozzles that produces larger droplets and that are optimally air inducted with narrow fan angles
  • Keep your water rates high
  • Narrow nozzle spacing
  • Slower travel speeds
  • Minimise the boom height but ensure at least a double overlap

For more information on sprayer setup for retained stubble systems make sure that you follow the link below.

How to set up my sprayer?

fallow spraying


Pre-emergent herbicides and windrow burning

With herbicide resistance levels on the rise growers have introduced windrow burning as a tool within their integrated weed management strategy. Windrow burning concentrates the previous years stubble and weed seeds into a narrow band that can be burnt.

If pre-emergent herbicides are used in conjunction with this technique then it may be worth considering the following points.

  • If the pre-emergent herbicide is applied over the top of the windrow before its burnt will result in little herbicide coming into contact with the soil.
  • Ensure that a hot burn is achieved in order to effectively control the weeds within the windrow. A hot burn will also minimise the degree of charcoal left over for herbicides to potentially bind to .
  • Herbicide is capable of binding tightly to ash as well as charcoal so where there are high levels of ash it may be worth waiting for a rainfall event to occur. The rainfall event will disperse the ash thus maximising herbicide soil contact.


The process of photodegredation occurs when the herbicide undergoes a chemical reaction in the presence of sunlight. This reaction breaks down the herbicide and the herbicide is lost to the weed control system. Under Australian conditions incorporation or rainfall events are usually sufficient for taking the herbicide into the soil profile. However if a herbicide is sprayed onto a dry soil or stubble during the summer period and there isn’t a follow up rainfall event or cultivation herbicides losses from the system may be the result.


A large portion of the pre-emergent herbicides that are used in Australia are considered to be volatile. These herbicides transition into a gaseous phase after application if the herbicide isnt incorporated. If the herbicide label says that it needs to be incorporated within 24 hours it doesnt mean that volatilisation wont occur until the 24 hour point. Volatalisation starts occuring immediately after application if left on the soil surface. With this in mind its important to incorporate the herbicide as soon as possible in order to maximnise the effects.

plowing the paddock



Incorporation is an essential component as volatilisation and photodegredation are capable of breaking down a herbicide to a point where its rendered innefective. The process of incorporation can happen in one of four ways.

1# Full cut mechanical  incorporation 

before the adoption of minimum till systems full cut incorporation was considered to be common practice. This process is usually done by a harrows or an offset disk. This is the best method for successfully incorporating highly volatile products.

2# Incorporation by sowing

This method is quite popular within minimum till systems as there is minimal disturbance. The seeder is setup so that it throws a small amount of treated soil out of the sowing furrow and into the inter-row to successfully  cover the herbicide. This process will only work on 25-30cm row spacing’s. When setting up the seeder make sure that there is adequate soil throw into the inter row to maintain herbicide coverage.

3# Irrigation

Irrigation is an effective method of incorporating pre-emergent herbicides into the soil assuming there is adequate water in relation to the soil type for infiltration to occur.

4# Rainfall

Rainfall is a popular method for incorporation during the fallow period. If adequate rainfall doesn’t occur after application alternative methods of incorporation may need to be applied in order to minimise the degree of herbicide breakdown.

Keep in mind with these incorporation methods that the main aim is to minimise the amount of exposure that the pre-emergent herbicide has to the sun.


  • When your looking to apply a pre-emergent herbicide there is a few things to keep in mind.
  • How volatile is the herbicide and is it susceptible to photo degradation?
  • How soluble is the herbicide?
  • How tightly will the herbicide bind to stubble and organic matter?
  • How will I incorporate the herbicide and what is my backup plan if the first method fails?

If you would like some additional information feel free to down load my reference from the link below. If you have any questions make sure that you leave them in the comments section or send us a message on facebook and twitter.

GRDC- Pre-emergent herbicides